1. Field of the Invention
This invention relates to pressure-sensitive adhesives, articles comprising substrates or backings coated with such adhesives, a hot-melt process for preparing such adhesive-coated backings, and the use of such articles. More specifically, this invention relates to pressure-sensitive adhesives having a partially oriented and partially crystallized elastomer component which, in a particularly preferred embodiment, imparts anisotropic peel behavior to the adhesive.
2. Description of the Related Art
Pressure-sensitive adhesives ("PSAs") are typically provided in the form of a tacky adhesive coating that is disposed on a backing (e.g., a tape or a sheet made of polymeric film, metal foil, paper, cloth, release liner, etc.). Such adhesives are useful for adhesively bonding two surfaces together with light finger-pressure, so as to form a relatively weak bond that usually can be easily broken by peeling the adhesive-coated tape or sheet to remove it from the surface to which it is attached. Preferably the adhesive-coated tape or sheet can be removed without damaging the surface and without leaving adhesive residue behind. A PSA article commonly used today is Scotch.TM. brand masking tape made by the 3M Company, which began using rubber-based PSAs for such articles in the late 1920s.
A solvent-free, hot-melt process for preparing a PSA from a tackified non-thermoplastic elastomer, such as natural rubber, polyisobutylene, and other hydrocarbon elastomers, is described in the PCT international patent application (of the assignee hereof) published May 26, 1994 as WO 94/11175. The process uses a continuous compounding device that has a sequence of alternating conveying zones and processing zones which masticate the elastomer and mix the elastomer, tackifier, and adjuvants to form the adhesive. Molten adhesive may be pumped through a coating die in the form of a thin film and directly onto a support which preferably comprises a moving web that passes around a heated coating roll.
Japanese kokai patent application no. HEI 71995!-18227, published Jan. 20, 1995, describes anisotropic adhesive material having an adhesive layer on at least one side of a base having anisotropic flexibility and corrugated or cord-like structure.
U.S. Pat. No. 5,156,911 (Stewart), issued Oct. 20, 1992, discloses skin-activated, temperature-sensitive adhesive assemblies. In one embodiment there is an adhesive that is substantially non-tacky at or below room temperature, becomes aggressively tacky at skin temperature, but reverts to its substantially non-tacky condition upon cooling (e.g., by applying ice or a cold pack). Regardless of the embodiment, the adhesive includes a crystallizable polymer that may be crosslinked. Similar materials are described by R. Clarke et al. in "Temperature Switchable Pressure Sensitive Adhesives," Adhesives Age, September, 1993, pp. 39-41.
There is a desire for adhesives that have anisotropic peel behavior (i.e., different adhesion when peeled in different directions). Such adhesives would be useful in many applications (e.g., graphic application tapes and other uses described more fully below). If an adhesive article having such properties could be provided independent of a backing, substantial flexibility in backing selection would be possible. It would also be desirable if a known adhesive composition could be manufactured in a manner to provide such properties. There is also a desire for adhesives having heat activatable tack as these would be useful in situations where low initial tack is advantageous, especially if known adhesive compositions can be manufactured in a manner to provide such properties.